Magnetic recording media are employed in a wide variety of applications, including identity cards, credit cards, banking cards, parking permits, hotel key cards, tollgate cards, data tapes, and floppy disks. In these and similar applications, it is desirable to provide a recording medium that both minimizes unintended erasure and maximizes storage capacity (bit density).
A magnetic recording media requires adequately high coercivity (a measure of magnetic field strength of a magnetized substance and of its resistance to demagnetization) to minimize accidental loss of stored data. In magnetic stripe cards, for example, accidental erasures associated with low coercivity of the recording medium account for 60% of card failures.
A magnetic media also preferably has relatively low magnetic interaction (a measure of the degree of magnetic interaction of one point on the media with adjacent areas on the media). Lower magnetic interaction allows increased bit density, providing increased data storage capacity in the same space.
Increases in storage capacity could provide extended capabilities in devices employing magnetic media. In the case of magnetic stripe cards, because a standard banking credit card has only 140 bytes of storage, a typical consumer uses several separate magnetic stripe cards. Larger storage capacity would allow the combination of several cards into one multifunctional magnetic card. New functions not currently performed by magnetic cards could even be added to such a card.
Magneto-plumbite ferrite particles have been used in magnetic recording media. Magneto-plumbite ferrite particles have the advantage of relatively high coercivity (e.g., anywhere from 300 to more than 3500 Oe), and have shown a corresponding greater resistance to erasure than media using typical acicular (needle-like) metal particles.
But magneto-plumbite ferrite particles have the disadvantage of tending to group together in clumps or stacks during general production, processing, and handling of the particles. Additionally, in the production of a magnetic layer employing magneto-plumbite ferrite particles, the still-wet layer is subjected to a magnetic field to orient the particles. This magnetic orientation process significantly increases particle grouping of magneto-plumbite ferrite particles. Groups of particles tends to act magnetically as a single particle, resulting in uneven and larger-than-desired effective particle sizes and increased magnetic interaction. Such groups of particles produce uneven magnetic properties in a magnetic media, resulting in high media noise and correspondingly lower maximum bit densities. To increase the storage capacity of a magnetic media, the density of such defects must be decreased in order to maintain an adequate media signal-to-noise ratio.
Grouping of particles during particle preparation can result in a small amount of strongly bonded clumps or stacks (agglomerates) remaining in the prepared powder. Such agglomerates do not disperse during the milling process for preparation of a magnetic paint. After the milling process, these non-dispersed particles clog filter pores during the filtering process, decreasing filter efficiency. The non-dispersed particles can also contribute to defects such as pinhole, stain, and rough surface on the top surface of the magnetic layer. These defects cause various problems, such as media noise (including dropout and signal spike), collision between head and media, etc.
At CardTech/SecurTech in 1996, Eastman Kodak Company announced the Intelligent Magnetics card system, having comparable capabilities to a chip (or smart) card but at much less cost. The MR (magneto resistive) head used for high-density storage applications such as the Intelligent Magnetics card system is much more sensitive than inductive heads used in current card systems. Consequently, a magnetic card for such a system requires a small concentration of magnetic particles uniformly dispersed throughout the whole card. If there are clumps or stacks of non-dispersed magnetic particles in a card, a magnetic field higher than that of the surrounding magnetic particles will be generated, resulting in a signal spike. In a card designed for high storage capacity, uniformity of dispersion is critical to obtaining a stable signal from the card.
For all of the above reasons, it is desirable to minimize particle grouping and maximize the uniformity of dispersion of magneto-plumbite ferrite particles for use in magnetic recording media.